The invention relates to flow-through capacitors for deionizing solutions, e.g., aqueous solutions, with improved operation at concentrated solutions, including such applications as low energy desalination of seawater. Technologies to deionize water include electrodeionization and flow-through capacitors. The term electrodeionization, including electrodialysis and continuous electrodeionization, has traditionally referred to a process or device that uses electrodes to transform electronic current into ionic current by oxidation-reduction reactions in anolyte and catholyte compartments located at the anodes and cathodes. Traditionally, ionic current has been used for deionization in ion-depleting compartments, and neither the anolyte chambers, the catholyte chambers nor the oxidation-reduction products have participated in the deionization process. In order to avoid contamination and to allow multiple depletion compartments between electrodes, the ion-concentrating and ion-depleting compartments were generally separated from the anolyte and catholyte compartments. To minimize formation of oxidation-reduction products at the electrodes, electrodeionization devices typically comprise multiple layers of ion-concentrating and ion-depleting compartments, bracketed between pairs of end electrodes.
One disadvantage of prior art systems is the energy loss resulting from using multiple compartment layers between electrodes, thereby creating an electrical resistance. This is generally true of prior art electrodeionization devices and is one characteristic that differentiates them from flow-through capacitors.
Flow-through capacitors differ in a number of other ways from electrodeionization devices as well. One difference is that flow-through capacitors purify water without oxidation-reduction reactions. The electrodes electrostatically adsorb and desorb contaminants, so that the electrode (anode and cathode) compartments participate directly in deionization and are located within one or both of the ion-depleting and ion-concentrating compartments. The anolyte and catholyte are partly or largely contained within a porous electrode. Electronic current is generally not transmuted by an oxidation-reduction reaction. Instead, charge is transferred by electrostatic adsorption.
However, flow-through capacitors of the prior art become energy inefficient and impractical at high ion or contaminant concentrations. The reason for this is due to the pore volume in the electrodes. Dissolved counterion salts present in the pore volume adsorb onto the electrodes, whereas pore volume coion salts are expelled from the electrodes. This has a doubly deleterious effect. Counterions occupy capacitance within the electrode. This amount of charge-holding capacitance is therefore unavailable for purification of ions from the feed water purification stream. Coions expelled from the electrodes enter the feed water purification stream and contaminate it with additional ions. This effect becomes worse with increased concentration. The flow-through capacitor is typically regenerated into liquid of the feed concentration. When purifying a concentrated liquid, ions are passively brought over into the pores prior to application of a voltage or electric current. Once voltage is applied, these ions are simultaneously adsorbed and expelled during the purification process. Purification can only occur when an excess of feed ions, over and above the pore volume ions, are adsorbed by the electrodes. This puts an upper practical limit on the economy of the flow-through capacitor, typically in the range of approximately 2500 to 6000 parts per million (ppm). The flow-through capacitor of the prior art requires both slower flow rates and higher energy usage. Beyond 6000 ppm, the energy usage required is typically more than 1 joule per coulomb of dissolved ions, making prior art flow-through capacitors too energy intensive to be practical. Seawater, which has ion concentrations of approximately 35,000 ppm, becomes impractical to deionize due to energy inefficiency caused by these pore volume losses. Pore volume losses occur at all concentrations but get worse at higher concentrations. Another way to describe pore volume losses is that they cause diminished ionic efficiency. Ionic efficiency is defined as the ratio of coulombs of ions purified to coulombs of electrons utilized.
Thus, a need exists to improve the ionic and energy efficiency of flow-through capacitors, particularly when treating solutions with ion concentrations in excess of 2500 ppm. A further need exists for a flow through capacitor to purify solutions with an energy usage of less than 1 Joule per Coloumb of purified ionic charge. Ionic efficiency is the coulombs of ionic charge purified per coulombs of electrons used, and should be 50% or more.